Thanks JM, if there were a pair of good condition frames available over your way, I would not bat an eyelid at paying a pretty penny to get them over here.

I was concerned that Cat in their usual wisdom used some wacky c-channel size for the frame rails (mine measured 5" on the base, 1.885 on one leg which is a standard size (C5x9) but the bottom leg is 2.450 which is not a standard size, however I think I can use the C5 x 9 and then maybe add a 1/4 plate to the bottom leg for a bit more beef. I know it's not the same as making the leg longer by 1/2" but I think welding an extra 1/2" on might not do anything because maybe the welding softens up the existing steel a bit. Anyway, I think brand new C5x9 is going to be way better than what I currently have

Thanks for this info Bob. My channels don't have that plate on the inside of the channel. There are just the pivot shaft tube, the curved piece beneath the recoil spring, and then it has three lateral braces which I think would take care of a lot of shrinkage from the curved piece. I was thinking of using a cutoff blade to get the curved piece to a regular size then running a bead along the sides to get it "back to size". For the channel leg size, it only matters for the slides on the idler mounts to sit properly, so the size of the bottom leg doesn't matter as regards that. I presume Cat just had it made bigger for additional strength on the idler forks.
When I studied the frames a bit more, the plate that sits forward of the pivot shaft tube could easily be approximated using pieces of flat - it doesn't have to be a pressed piece. Technically the curved piece under the recoil spring could also be made from flat but I'll wager my guy has a spare piece of round he could put in the press.
I'll still patch up the old frame because I know that building a new set will take longer than we have til the spring plow days so it'll keep the tractor mobile in the meantime until the new one's ready

Agreed they look like crap but I was pushing brush and small trees over pretty hard with it and they held up, so I think they're a lot stronger than they look : )